The present invention relates to a temperature-independent variable-current source.
As is known, the need is often felt to generate a current which is correlated to a variable external voltage but is practically insensitive to the temperature variations which may affect the integrated circuit in which the souce is physically comprised. It is sometimes also required that the variation range of the produced current be fixed and preset, thus ensuring that the value of the current is always comprised between a minimum value and a maximum value.
Current sources adapted to generate a current which is variable as a function of an input voltage are known in variousd forms. For example, FIG. 1 illustrates a very simple diagram implementing a variable current source. In this circuit, which comprises a current mirror formed by a pair of transistors T.sub.1 and T.sub.2 (of which T.sub.1 is diodeconnected) both of which have their emitters connected to the power supply V.sub.CC, their bases connected to one another and their collectors which respectively define, through the resistor R, the input (contact pad 1) receiving the variable input voltage V.sub.IN and the output feeding the output current I.sub.O, the following is true: ##EQU1## where V.sub.BE1 is the base-emitter drop of the transistor T.sub.1.
The mirror structure, with T.sub.1 =T.sub.2, forces I.sub.O =I.sub.X so that by varying the input voltage V.sub.IN the output current I.sub.O varies accordingly.
However, since V.sub.BE1 and R are temperature-dependent, I.sub.O has the following thermal drift: ##EQU2## wherein the input voltage V.sub.IN is assumed to be temperature-independent. This equation generally yields a non-zero result, so that the described structure supplies an output current the value whereof varies according to the temperature.
Another structure used to generate variable currents is shown in FIG. 2, and comprises a pair of transistors T.sub.3 and T.sub.4, the emitters whereof are coupled through the resistor R'; the bases of said transistors are respectively connected to the input voltage V.sub.IN and to a reference voltage V.sub.REF. The collector of T.sub.4 is furthermore connected to the supply voltage V.sub.CC, the emitter of T.sub.3 is connected to a fixed current source I and its collector defines the output which supplies the current I.sub.O. The following relations are true for this circuit: ##EQU3## wherein V.sub.BE3 and V.sub.BE4 are the base-emitter drops of T.sub.3 and T.sub.4. By rewritting I.sub.Y, the following is obtained: ##EQU4## inserting the law which links the collector current to the base-emitter drop of T.sub.3 and T.sub.4.
The temperature-dependence of I.sub.Y, and therefore of I.sub.O, is thus evident, so that the desired temperature-independence cannot be achieved even with the structure shown in FIG. 2.